[riscv-port-jdk17u:riscv-port] RFR: 8296435: RISC-V: Small refactoring for increment/decrement

[Dingli Zhang]

Please review this backport to riscv-port-jdk17u.
Backport of [JDK-8296435](https://bugs.openjdk.org/browse/JDK-8296435).
The original patch cannot be directly applied because jdk17u has no [JDK-8286301](https://bugs.openjdk.org/browse/JDK-8286301).

Testing:
- Tier1-3 passed without new failure on unmacthed (release).

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Commit messages:
 - 8296435: RISC-V: Small refactoring for increment/decrement

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/48/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=48&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296435
  Stats: 36 lines in 3 files changed: 0 ins; 2 del; 34 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/48.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u.git pull/48/head:pull/48

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/48